Evaluation of cytotoxicity features of antimicrobial peptides with potential to control bacterial diseases of citrus

Antimicrobial Peptides: Classification, Mechanism, and Application in Plant Disease Resistance

Antimicrobial peptides (AMPs) are a class of alkaline, small molecules found widely in nature. This article surveys the classifications of AMPs, delving into their modes of action and their role in controlling significant plant diseases caused by bacteria, viruses, and fungi. It also explores the prospects and challenges in this field, aiming to provide insights for enhancing crop disease resistance, ensuring food security, deepening the understanding of pathogen mechanisms, and protecting ecological balance.

Harnessing bacterial antimicrobial peptides: a comprehensive review on properties, mechanisms, applications, and challenges in combating antimicrobial resistance

Antimicrobial resistance (AMR) is a significant global health concern due to the persistence of pathogens and the emergence of resistance in bacterial infections. Bacterial-derived antimicrobial peptides (BAMPs) have emerged as a promising strategy to combat these challenges. Known for their diversity and multifaceted nature, BAMPs are notable bioactive agents that exhibit potent antimicrobial activities against various pathogens. This review explores the intricate properties and underlying mechanisms of BAMPs, emphasizing their diverse applications in addressing AMR. Additionally, the review investigates the mechanisms, analyses the challenges in utilizing BAMPs effectively, and examines their potential applications and associated deployment challenges providing comprehensive insights into how BAMPs can be harnessed to combat AMR across different domains. The significance of this review lies in highlighting the potential of BAMPs as transformative agents in combating AMR, offering sustainable and eco-friendly solutions to this pressing global health challenge.

Novel Leech Antimicrobial Peptides, Hirunipins: Real-Time 3D Monitoring of Antimicrobial and Antibiofilm Mechanisms Using Optical Diffraction Tomography

Antimicrobial peptides (AMPs) are promising agents for treating antibiotic-resistant bacterial infections. Although discovering novel AMPs is crucial for combating multidrug-resistant bacteria and biofilm-related infections, their clinical potential relies on precise, real-time evaluation of efficacy, toxicity, and mechanisms. Optical diffraction tomography (ODT), a label-free imaging technology, enables real-time visualization of bacterial morphological changes, membrane damage, and biofilm formation over time. Here, a computational analysis of the leech transcriptome using an advanced AI-based peptide screening strategy with ODT to identify potential AMPs is employed. Among the 19 potential AMPs identified, hirunipin 2 demonstrates potent antibacterial activity, low mammalian cytotoxicity, and minimal hemolytic effects. It demonstrates efficacy comparable to melittin, resistance to physiological salts and human serum, and a low likelihood of inducing bacterial resistance. Microscopy and 3D-ODT confirm its disruption of bacterial membranes and intracellular aggregation, leading to cell death. Notably, hirunipin 2 effectively inhibits biofilm formation, eradicates preformed biofilms, and synergizes with antibiotics against multidrug-resistant Acinetobacter baumannii (MDRAB) by enhancing membrane permeability. Additionally, hirunipin 2 significantly suppresses pro-inflammatory cytokine expression in LPS-stimulated macrophages, highlighting its anti-inflammatory properties. These findings highlight hirunipin 2 as a strong candidate for developing novel antibacterial, anti-inflammatory, and antibiofilm therapies, particularly against multidrug-resistant bacterial infections.

Genome-Guided Identification and Characterisation of Broad-Spectrum Antimicrobial Compounds of Bacillus velezensis Strain PD9 Isolated from Stingless Bee Propolis

The emergence of multidrug-resistant pathogens presents a significant global health challenge, which is primarily fuelled by overuse and misuse of antibiotics. Bacteria-derived antimicrobial metabolites offer a promising alternative strategy for combating antimicrobial resistance issues. Bacillus velezensis PD9 (BvPD9), isolated from stingless bee propolis, has been reported to have antibacterial activities against methicillin-resistant Staphylococcus aureus (MRSA). This study aimed to characterise and identify the antimicrobial compounds (AMCs) synthesised by BvPD9 through integration of genome mining and liquid chromatography-mass spectrometry (LC-MS) analysis. The whole-genome sequence of BvPD9 contained 4,263,351 base pairs and 4101 protein-coding sequences, with 12 potential AMC biosynthetic gene clusters. Comparative genomic analysis highlighted the unique profile of BvPD9 that possesses the largest number of unknown proteins, indicating significant potential for further exploration. The combined genomics-metabolic profiling uncovered five AMCs in BvPD9 extract, including bacillibactin, bacilysin, surfactin A, fengycin A, and bacillomycin D. The extract exhibited a broad antibacterial spectrum against 25 pathogenic bacteria, including both Gram-positive and Gram-negative bacteria, with the lowest minimum inhibitory concentration (MIC, 0.032 mg/ml) against S. epidermidis ATCC 12228, and the lowest minimum bactericidal concentration (MBC; 0.128 mg/ml) against MRSA ATCC 700699 and Aeromonas hydrophilia. The robust stability of BvPD9 extract was demonstrated at high temperatures, over a wide range of pH conditions (6 to 12) and in the presence of various hydrolytic enzymes. Additionally, the extract showed 50% haemolytic and cytotoxicity activity at 0.158 and 0.250 mg/ml, respectively. These characteristics suggest potential applications of BvPD9 metabolites for tackling antimicrobial resistance and its applicability across diverse industries.

Molecular Interactions of the Antimicrobial Peptide Tritrpticin with Mixed Nanoaggregates: A Fluorescence Spectroscopy Study

INTRODUCTION

Tritrpticin (TRP3) is a peptide belonging to the cathelicidin family and has a broad spectrum of antimicrobial activity. However, this class of biomolecules can be easily degraded in the body, making it necessary to use an efficient transport system. The ability to form stable nanostructures from the interaction of glycyrrhizin saponin with the pluronic polymer F127 was demonstrated, forming mixed biopolymeric micelles, highly promising as drug carriers.

OBJECTIVE

The present work sought to understand the physicochemical interaction of the antimicrobial peptide TRP3 with the mixed polymeric micelle made from pluronic F127 and the saponin glycyrrhizin.

METHODS

The interaction of tritrpticin with mixed nanostructured micelles was evaluated through fluorescence spectroscopy and fluorescence quenching with acrylamide. The experiments were performed at room temperature (25 ± 1°C), adopting an excitation wavelength set to 280 nm and emission between 300 and 500 nm, with a slit of 5 nm.

RESULTS

The interaction of the cationic peptide tritrpticin with the mixed biopolymeric micelles was observed through the blue shift of the fluorescence emission to shorter wavelengths, proving the change of tryptophan to a more hydrophobic environment. Through the fluorescence suppression technique, it was possible to indicate the location of the peptide in the mixed micelles, proving tritrpticin to be partially inserted inside them.

CONCLUSION

It was concluded that tritrpticin interacted with mixed nanostructured micelles, forming a promising system for biotechnological applications.

Molecular Mechanisms of Bacterial Resistance to Antimicrobial Peptides in the Modern Era: An Updated Review

Antimicrobial resistance (AMR) poses a serious global health concern, resulting in a significant number of deaths annually due to infections that are resistant to treatment. Amidst this crisis, antimicrobial peptides (AMPs) have emerged as promising alternatives to conventional antibiotics (ATBs). These cationic peptides, naturally produced by all kingdoms of life, play a crucial role in the innate immune system of multicellular organisms and in bacterial interspecies competition by exhibiting broad-spectrum activity against bacteria, fungi, viruses, and parasites. AMPs target bacterial pathogens through multiple mechanisms, most importantly by disrupting their membranes, leading to cell lysis. However, bacterial resistance to host AMPs has emerged due to a slow co-evolutionary process between microorganisms and their hosts. Alarmingly, the development of resistance to last-resort AMPs in the treatment of MDR infections, such as colistin, is attributed to the misuse of this peptide and the high rate of horizontal genetic transfer of the corresponding resistance genes. AMP-resistant bacteria employ diverse mechanisms, including but not limited to proteolytic degradation, extracellular trapping and inactivation, active efflux, as well as complex modifications in bacterial cell wall and membrane structures. This review comprehensively examines all constitutive and inducible molecular resistance mechanisms to AMPs supported by experimental evidence described to date in bacterial pathogens. We also explore the specificity of these mechanisms toward structurally diverse AMPs to broaden and enhance their potential in developing and applying them as therapeutics for MDR bacteria. Additionally, we provide insights into the significance of AMP resistance within the context of host-pathogen interactions.

Specifically targeting antimicrobial peptides for inhibition of Candidatus Liberibacter asiaticus

AIMS

Huanglongbing (citrus greening) is a plant disease putatively caused by the unculturable Gram-negative bacterium Candidatus Liberibacter asiaticus (CLas), and it has caused severe damage to citrus plantations worldwide. There are no definitive treatments for this disease, and conventional disease control techniques have shown limited efficacy. This work presents an in silico evaluation of using specifically targeting anti-microbial peptides (STAMPs) consisting of a targeting segment and an antimicrobial segment to inhibit citrus greening by inhibiting the BamA protein of CLas, which is an outer membrane protein crucial for bacterial viability.

METHODS AND RESULTS

Initially, a set of peptides with a high affinity toward BamA protein were screened and evaluated via molecular docking and molecular dynamics simulations and were verified in vitro via bio-layer interferometry (BLI). In silico studies and BLI experiments indicated that two peptides, HASP2 and HASP3, showed stable binding to BamA. Protein structures for STAMPs were created by fusing known anti-microbial peptides (AMPs) with the selected short peptides. The binding of STAMPs to BamA was assessed using molecular docking and binding energy calculations. The attachment of high-affinity short peptides significantly reduced the free energy of binding for AMPs, suggesting that it would make it easier for the STAMPs to bind to BamA. Efficacy testing in vitro using a closely related CLas surrogate bacterium showed that STAMPs had greater inhibitory activity than AMP alone.

CONCLUSIONS

In silico and in vitro results indicate that the STAMPs can inhibit CLas surrogate Rhizobium grahamii more effectively compared to AMPs, suggesting that STAMPs can achieve better inhibition of CLas, potentially via enhancing the site specificity of AMPs.

Efficiency of the alpha-hairpinin SmAMP-X gene promoter from Stellaria media plant depends on selection of transgenic approach

The antimicrobial activity of the alpha-HAIRPININ ANTIMICROBIAL PEPTIDE X (SmAMP-X gene, GenBank acc. No. HG423454.1) from Stellaria media plant has been shown in vitro. Here, we isolated the SmAMP-X gene promoter and found two genomic sequences for the promoter (designated pro-SmAMP-X and pro-SmAMP-X-Ψ2) with 83% identity in their core and proximal regions. We found that the abilities of these promoters to express the uidA reporter and the nptII selectable marker differ according to the structural organization of T-DNA in the binary vector used for plant transformation. Analysis of Agrobacterium-infiltrated Nicotiana benthamiana leaves, transgenic Arabidopsis thaliana lines, and transgenic Solanum tuberosum plants revealed that both promoters in the pCambia1381Z and pCambia2301 binary vectors generate 42-100% of the ß-glucuronidase (GUS) activity generated by the CaMV35S promoter. According to 5'-RACE (rapid amplification of cDNA ends) analysis, both plant promoters are influenced by the CaMV35S enhancer used to express selectable markers in the T-DNA region of pCambia1381Z and pCambia2301. The exclusion of CaMV35S enhancer from the T-DNA region significantly reduces the efficiency of pro-SmAMP-X-Ψ2 promoter for GUS production. Both promoters in the pCambia2300 vector without CaMV35S enhancer in the T-DNA region weakly express the nptII selectable marker in different tissues of transgenic N. tabacum plants and enable selection of transgenic cells in media with a high concentration of kanamycin. Overall, promoter sequences must be functionally validated in binary vectors lacking CaMV35S enhancer.

Evaluation of Novel HLM Peptide Activity and Toxicity against Planktonic and Biofilm Bacteria: Comparison to Standard Antibiotics

BACKGROUND

Antibiotic resistance is one of the main concerns of public health, and the whole world is trying to overcome such a challenge by finding novel therapeutic modalities and approaches. This study has applied the sequence hybridization approach to the original sequence of two cathelicidin natural parent peptides (BMAP-28 and LL-37) to design a novel HLM peptide with broad antimicrobial activity.

METHODS

The physicochemical characteristics of the newly designed peptide were determined. As well, the new peptide's antimicrobial activity (Minimum Inhibitory Concentration (MIC), Minimum Bacterial Eradication Concentration (MBEC), and antibiofilm activity) was tested on two control (Staphylococcus aureus ATCC 29213, Escherichia coli ATCC 25922) and two resistant (Methicillin-resistant Staphylococcus aureus (MRSA) ATCC BAA41, New Delhi metallo-beta- lactamase-1 Escherichia coli ATCC BAA-2452) bacterial strains. Furthermore, synergistic studies have been applied to HLM-hybridized peptides with five conventional antibiotics by checkerboard assays. Also, the toxicity of HLM-hybridized peptide was studied on Vero cell lines to obtain the IC50 value. Besides the percentage of hemolysis action, the peptide was tested in freshly heparinized blood.

RESULTS

The MIC values for the HLM peptide were obtained as 20, 10, 20, and 20 μM, respectively. Also, the results showed no hemolysis action, with low to slightly moderate toxicity action against mammalian cells, with an IC50 value of 10.06. The Biomatik corporate labs, where HLM was manufactured, determined the stability results of the product by Mass Spectrophotometry (MS) and High-performance Liquid Chromatography (HPLC) methods. The HLM-hybridized peptide exhibited a range of synergistic to additive antimicrobial activities upon combination with five commercially available different antibiotics. It has demonstrated the biofilm-killing effects in the same concentration required to eradicate the control strains.

CONCLUSION

The results indicated that HLM-hybridized peptide displayed a broad-spectrum activity toward different bacterial strains in planktonic and biofilm forms. It showed synergistic or additive antimicrobial activity upon combining with commercially available different antibiotics.

Functional Peptides for Plant Disease Control

Plant disease control requires novel approaches to mitigate the spread of and losses caused by current, emerging, and re-emerging diseases and to adapt plant protection to global climate change and the restrictions on the use of conventional pesticides. Currently, disease management relies mainly on biopesticides, which are required for the sustainable use of plant-protection products. Functional peptides are candidate biopesticides because they originate from living organisms or are synthetic analogs and provide novel mechanisms of action against plant pathogens. Hundreds of compounds exist that cover an extensive range of activities against viruses, bacteria and phytoplasmas, fungi and oomycetes, and nematodes. Natural sources, chemical synthesis, and biotechnological platforms may provide peptides at large scale for the industry and growers. The main challenges for their use in plant disease protection are (a) the requirement of stability in the plant environment and counteracting resistance in pathogen populations, (b) the need to develop suitable formulations to increase their shelf life and methods of application, (c) the selection of compounds with acceptable toxicological profiles, and (d) the high cost of production for agricultural purposes. In the near future, it is expected that several functional peptides will be commercially available for plant disease control, but more effort is needed to validate their efficacy at the field level and fulfill the requirements of the regulatory framework.

The pharmacokinetic-pharmacodynamic modelling framework as a tool to predict drug resistance evolution

Pharmacokinetic-pharmacodynamic (PKPD) models, which describe how drug concentrations change over time and how that affects pathogen growth, have proven highly valuable in designing optimal drug treatments aimed at bacterial eradication. However, the fast rise of antimicrobial resistance calls for increased focus on an additional treatment optimization criterion: avoidance of resistance evolution. We demonstrate here how coupling PKPD and population genetics models can be used to determine treatment regimens that minimize the potential for antimicrobial resistance evolution. Importantly, the resulting modelling framework enables the assessment of resistance evolution in response to dynamic selection pressures, including changes in antimicrobial concentration and the emergence of adaptive phenotypes. Using antibiotics and antimicrobial peptides as an example, we discuss the empirical evidence and intuition behind individual model parameters. We further suggest several extensions of this framework that allow a more comprehensive and realistic prediction of bacterial escape from antimicrobials through various phenotypic and genetic mechanisms.

Development of a Novel Antibacterial Peptide, PAM-5, via Combination of Phage Display Selection and Computer-Assisted Modification

Antibacterial peptides (ABPs) have been proposed as potential candidates for alternative antibacterial agents due to the extensive dissemination of antibiotic resistance. However, ABP isolation from natural resources can be tedious without consistent yield. Moreover, many natural ABPs are not developed for clinical application due to potential toxicity to mammalian cells. Therefore, the objective of this study was to develop a potent ABP with minimal toxicity via phage display selection followed by computer-assisted modification. Briefly, a 12-mer phage-displayed peptide library was used to isolate peptides that bound to the cell surface of Pseudomonas aeruginosa with high affinity. The affinity-selected peptide with the highest selection frequency was modified to PAM-5 (KWKWRPLKRKLVLRM) with enhanced antibacterial features by using an online peptide database. Using in vitro microbroth dilution assay, PAM-5 was shown to be active against a panel of Gram-negative bacteria and selected Gram-positive bacteria. Interestingly, the peptide was stable in human plasma by exhibiting a similar bactericidal effect via ex vivo assay. Scanning electron microscopy and SYTOX Green uptake assay revealed that PAM-5 was able to cause membrane disruption and permeabilization of the bacteria. Additionally, the peptide was also able to bind to bacterial DNA as demonstrated by gel retardation assay. In the time-kill assay, PAM-5 was shown to kill the bacteria rapidly in 10 min. More importantly, PAM-5 was non-cytotoxic to Vero cells and non-haemolytic to human erythrocytes at all concentrations tested for the antibacterial assays. Thus, this study showed that the combination of phage display screening and computer-assisted modification could be used to develop potent novel ABPs, and PAM-5 derived from these approaches is worth to be further elucidated for its potential clinical use.

Host-derived chimeric peptides clear the causative bacteria and augment host innate immunity during infection: A case study of HLB in citrus and fire blight in apple

Bacterial diseases cause severe losses in the production and revenue of many fruit crops, including citrus and apple. Huanglongbing (HLB) in citrus and fire blight in apple are two deadly diseases without any cure. In this article, we introduce a novel therapy for HLB and fire blight by enhancing the innate immunity of the host plants. Specifically, we constructed in silico a library of chimeras containing two different host peptides with observed or predicted antibacterial activity. Subsequently, we performed bactericidal and toxicity tests in vitro to select a few non-toxic chimeras with high antibacterial activity. Finally, we conducted ex planta studies to show that not only do the chimeras clear the causative bacteria from citrus leaves with HLB and from apple leaves with fire blight but they also augment the host's innate immunity during infection. This platform technology can be extended to design host-derived chimeras against multiple pathogenic bacteria that cause diseases in plants and animals of agricultural importance and in humans.

Antimicrobial Peptides Therapy: An Emerging Alternative for Treating Drug-Resistant Bacteria

Microbial resistance to antibiotics is an ancient and dynamic issue that has brought a situation reminiscent of the pre-antibiotic era to the limelight. Currently, antibiotic resistance and the associated infections are widespread and pose significant global health and economic burden. Thus, the misuse of antibiotics, which has increased resistance, has necessitated the search for alternative therapeutic agents for combating resistant pathogens. Antimicrobial peptides (AMPs) hold promise as a viable therapeutic approach against drug-resistant pathogens. AMPs are oligopeptides with low molecular weight. They have broad-spectrum antimicrobial activities against pathogenic microorganisms. AMPs are nonspecific and target components of microbes that facilitate immune response by acting as the first-line defense mechanisms against invading pathogenic microbes. The diversity and potency of AMPs make them good candidates for alternative use. They could be used alone or in combination with several other biomaterials for improved therapeutic activity. They can also be employed in vaccine production targeting drug-resistant pathogens. This review covers the opportunities and advances in AMP discovery and development targeting antimicrobial resistance (AMR) bacteria. Briefly, it presents an overview of the global burden of the antimicrobial resistance crisis, portraying the global magnitude, challenges, and consequences. After that, it critically and comprehensively evaluates the potential roles of AMPs in addressing the AMR crisis, highlighting the major potentials and prospects.

Synthetic Peptides against Plant Pathogenic Bacteria

The control of plant diseases caused by bacteria that seriously compromise crop productivity around the world is still one of the most important challenges in food security. Integrated approaches for disease control generally lack plant protection products with high efficacy and low environmental and health adverse effects. Functional peptides, either from natural sources or synthetic, are considered as novel candidates to develop biopesticides. Synthetic peptides can be obtained based on the structure of natural compounds or de novo designed, considering the features of antimicrobial peptides. The advantage of this approach is that analogues can be conveniently prepared, enabling the identification of sequences with improved biological properties. Several peptide libraries have been designed and synthetized, and the best sequences showed strong bactericidal activity against important plant pathogenic bacteria, with a good profile of biodegradability and low toxicity. Among these sequences, there are bacteriolytic or antibiofilm peptides that work against the target bacteria, plant defense elicitor peptides, and multifunctional peptides that display several of these properties. Here, we report the research performed by our groups during the last twenty years, as well as our ongoing work. We also highlight those peptides that can be used as candidates to develop novel biopesticides, and the main challenges and prospects.

Plant Antimicrobial Peptides (PAMPs): Features, Applications, Production, Expression, and Challenges

The quest for an extraordinary array of defense strategies is imperative to reduce the challenges of microbial attacks on plants and animals. Plant antimicrobial peptides (PAMPs) are a subset of antimicrobial peptides (AMPs). PAMPs elicit defense against microbial attacks and prevent drug resistance of pathogens given their wide spectrum activity, excellent structural stability, and diverse mechanism of action. This review aimed to identify the applications, features, production, expression, and challenges of PAMPs using its structure-activity relationship. The discovery techniques used to identify these peptides were also explored to provide insight into their significance in genomics, transcriptomics, proteomics, and their expression against disease-causing pathogens. This review creates awareness for PAMPs as potential therapeutic agents in the medical and pharmaceutical fields, such as the sensitive treatment of bacterial and fungal diseases and others and their utilization in preserving crops using available transgenic methods in the agronomical field. PAMPs are also safe to handle and are easy to recycle with the use of proteases to convert them into more potent antimicrobial agents for sustainable development.

The characteristics and roles of antimicrobial peptides as potential treatment for antibiotic-resistant pathogens: a review

The emergence of antibiotic-resistant bacteria has become a significant and ever-increasing threat to global public health, increasing both morbidity and mortality rates, and the financial burden on health services. Infection by drug-resistant bacteria is anticipated to contribute to the demise of almost 10 million people by the year 2050 unless a competent and effective response is devised to engage with this issue. The emergence and spread of resistance are commonly caused by the excessive or inappropriate use of antibiotics and substandard pharmaceuticals. It arises when pathogens adapt to different conditions and develop self-defence mechanisms. Currently, novel antimicrobial peptides (AMPs) have been reported to be the sole cure for some clinical cases of infectious diseases such as sepsis and skin infections, although these agents may, on occasion, require administration together with an adjunctive low-dose antibiotic. Although AMPs are a promising alternative form of anti-microbial therapy and easily applied in the medical sector, they still have limitations that should not be taken lightly. Hence, this review explores the characteristics, advantages and disadvantages of AMPs for their potential in treating antibiotic-resistant pathogens.

Antimicrobial Peptides With Antibiofilm Activity Against Xylella fastidiosa

Xylella fastidiosa is a plant pathogen that was recently introduced in Europe and is causing havoc to its agriculture. This Gram-negative bacterium invades the host xylem, multiplies, and forms biofilm occluding the vessels and killing its host. In spite of the great research effort, there is no method that effectively prevents or cures hosts from infections. The main control strategies up to now are eradication, vector control, and pathogen-free plant material. Antimicrobial peptides have arisen as promising candidates to combat this bacterium due to their broad spectrum of activity and low environmental impact. In this work, peptides previously reported in the literature and newly designed analogs were studied for its bactericidal and antibiofilm activity against X. fastidiosa. Also, their hemolytic activity and effect on tobacco leaves when infiltrated were determined. To assess the activity of peptides, the strain IVIA 5387.2 with moderate growth, able to produce biofilm and susceptible to antimicrobial peptides, was selected among six representative strains found in the Mediterranean area (DD1, CFBP 8173, Temecula, IVIA 5387.2, IVIA 5770, and IVIA 5901.2). Two interesting groups of peptides were identified with bactericidal and/or antibiofilm activity and low-moderate toxicity. The peptides 1036 and RIJK2 with dual (bactericidal-antibiofilm) activity against the pathogen and moderate toxicity stand out as the best candidates to control X. fastidiosa diseases. Nevertheless, peptides with only antibiofilm activity and low toxicity are also promising agents as they could prevent the occlusion of xylem vessels caused by the pathogen. The present work contributes to provide novel compounds with antimicrobial and antibiofilm activity that could lead to the development of new treatments against diseases caused by X. fastidiosa.

Antimicrobial and Anti-Biofilm Peptide Octominin for Controlling Multidrug-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a serious nosocomial pathogen with multiple drug resistance (MDR), the control of which has become challenging due to the currently used antibiotics. Our main objective in this study is to determine the antibacterial and antibiofilm activities of the antimicrobial peptide, Octominin, against MDR A. baumannii and derive its possible modes of actions. Octominin showed significant bactericidal effects at a low minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of 5 and 10 µg/mL, respectively. Time-kill kinetic analysis and bacterial viability tests revealed that Octominin showed a concentration-dependent antibacterial activity. Field-emission scanning electron microscopy (FE-SEM) analysis revealed that Octominin treatment altered the morphology and membrane structure of A. baumannii. Propidium iodide (PI) and reactive oxygen species (ROS) generation assays showed that Octominin increased the membrane permeability and ROS generation in A. baumannii, thereby causing bacterial cell death. Further, a lipopolysaccharides (LPS) binding assay showed an Octominin concentration-dependent LPS neutralization ability. Biofilm formation inhibition and eradication assays further revealed that Octominin inhibited biofilm formation and showed a high biofilm eradication activity against A. baumannii. Furthermore, up to a concentration of 100 µg/mL, Octominin caused no hemolysis and cell viability changes in mammalian cells. An in vivo study in zebrafish showed that the Octominin-treated group had a significantly higher relative percentage survival (54.1%) than the untreated group (16.6%). Additionally, a reduced bacterial load and fewer alterations in histological analysis confirmed the successful control of A. baumannii by Octominin in vivo. Collectively, these data suggest that Octominin exhibits significant antibacterial and antibiofilm activities against the multidrug-resistant A. baumannii, and this AMP can be developed further as a potent AMP for the control of antibiotic resistance.

Understanding the mechanism of action of peptide (p-BthTX-I)2 derived from C-terminal region of phospholipase A2 (PLA2)-like bothropstoxin-I on Gram-positive and Gram-negative bacteria

Based on the antimicrobial activity of bothropstoxin-I (BthTX-I) and on the premise that a C-terminal peptide of Lys49 myotoxin can reproduce the antimicrobial activity of the parent protein, we aimed to study the mechanism of action of a peptide derived from the C-terminal region of the myotoxin BthTX-I [(p-BthTX-I)₂, sequence: KKYRYHLKPFCKK, disulfide-linked dimer] against Gram-positive and Gram-negative bacteria. Fluorescence quenching technique showed that the carboxyfluorescein labeled-peptide [CF-(p-BthTX-I)₂] when incubated with E. coli displayed a superior penetration activity than when incubated with S. aureus. Cell death induced by the peptide (p-BthTX-I)₂ showed a loss of membrane integrity in E. coli and S. aureus; however, the mechanisms of cell death were different, characterized by the presence of necrosis-like and apoptosis-like deaths, respectively. Scanning electron microscopy studies in E. coli and S. aureus showed morphological changes in the cells, with superficial deformities, appearance of wrinkles and bubbles, and formation of vesicles. Our results demonstrate that the mechanism of action of the peptide (p-BthTX-I)₂ is different in Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria. Knowledge of the mechanism of action of these peptides is important, since they are promising prototypes for new antimicrobial drugs.

Beneficial Impacts of Incorporating the Non-Natural Amino Acid Azulenyl-Alanine into the Trp-Rich Antimicrobial Peptide buCATHL4B

Antimicrobial peptides (AMPs) present a promising scaffold for the development of potent antimicrobial agents. Substitution of tryptophan by non-natural amino acid Azulenyl-Alanine (AzAla) would allow studying the mechanism of action of AMPs by using unique properties of this amino acid, such as ability to be excited separately from tryptophan in a multi-Trp AMPs and environmental insensitivity. In this work, we investigate the effect of Trp→AzAla substitution in antimicrobial peptide buCATHL4B (contains three Trp side chains). We found that antimicrobial and bactericidal activity of the original peptide was preserved, while cytocompatibility with human cells and proteolytic stability was improved. We envision that AzAla will find applications as a tool for studies of the mechanism of action of AMPs. In addition, incorporation of this non-natural amino acid into AMP sequences could enhance their application properties.

A Synergic Potential of Antimicrobial Peptides against Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa) is the pathogenic agent responsible for the bacterial canker of kiwifruit (BCK) leading to major losses in kiwifruit productions. No effective treatments and measures have yet been found to control this disease. Despite antimicrobial peptides (AMPs) having been successfully used for the control of several pathogenic bacteria, few studies have focused on the use of AMPs against Psa. In this study, the potential of six AMPs (BP100, RW-BP100, CA-M, 3.1, D4E1, and Dhvar-5) to control Psa was investigated. The minimal inhibitory and bactericidal concentrations (MIC and MBC) were determined and membrane damaging capacity was evaluated by flow cytometry analysis. Among the tested AMPs, the higher inhibitory and bactericidal capacity was observed for BP100 and CA-M with MIC of 3.4 and 3.4-6.2 µM, respectively and MBC 3.4-10 µM for both. Flow cytometry assays suggested a faster membrane permeation for peptide 3.1, in comparison with the other AMPs studied. Peptide mixtures were also tested, disclosing the high efficiency of BP100:3.1 at low concentration to reduce Psa viability. These results highlight the potential interest of AMP mixtures against Psa, and 3.1 as an antimicrobial molecule that can improve other treatments in synergic action.

Avian antimicrobial peptides: in vitro and in ovo characterization and protection from early chick mortality caused by yolk sac infection

Increasing antibiotic resistance is a matter of grave concern for consumers, public health authorities, farmers, and researchers. Antimicrobial peptides (AMPs) are emerging as novel and effective non-antibiotic tools to combat infectious diseases in poultry. In this study, we evaluated six avian AMPs including 2 truncated cathelicidins, [CATH-1(6-26) and CATH-2(1-15)], and 4 avian β-defensins (ABD1, 2, 6 and 9) for their bactericidal and immunomodulatory activities. Our findings have shown CATH-1(6-26) and ABD1 being the two most potent avian AMPs effective against Gram-positive and Gram-negative bacteria investigated in these studies. Moreover, CATH-1(6-26) inhibited LPS-induced NO production and exhibited dose-dependent cytotoxicity to HD11 cells. While, ABD1 blocked LPS-induced IL-1β gene induction and was non-toxic to HD11 cells. Importantly, in ovo administration of these AMPs demonstrated that ABD1 can offer significant protection from early chick mortality (44% less mortality in ABD1 treated group versus the control group) due to the experimental yolk sac infection caused by avian pathogenic Escherichia coli. Our data suggest that in ovo administration of ABD1 has immunomodulatory and anti-infection activity comparable with CpG ODN. Thus, ABD1 can be a significant addition to potential alternatives to antibiotics for the control of bacterial infections in young chicks.

Interaction of synthetic antimicrobial peptides of the Hylin a1 family with models of eukaryotic structures: Zwitterionic membranes and DNA

Antimicrobial peptides (AMPs) have been appointed as a possible alternative to traditional antibiotics in face of pathogens increasing resistance to conventional drugs. Hylin a1 (IFGAILPLALGALKNLIK), an AMP extracted from the skin secretion of a South American frog, Hypsiboas albopunctatus, was found to show a strong cytotoxicity against bacteria and fungus, but also a considerable hemolytic action. Considering the toxicity of the peptide in eukaryotic cells, this work focuses on investigating the effects of the interaction of the Hylin a1 analogues W⁶Hya1, D⁰W⁶Hya1 and K⁰W⁶Hya1 with models of eukaryotic structures, namely zwitterionic liposomes of dipalmitoyl phosphatidylcholine (DPPC) and calf-thymus DNA (CT DNA). Through intrinsic Trp fluorescence we determined that the peptide affinity for fluid DPPC bilayers follows the decreasing order: D⁰W⁶Hya1 (+2) > W⁶Hya1 (+3) » K⁰W⁶Hya1 (+4). Fluorescence data also indicate that the Trp residue in the more positively charged peptide, K⁰W⁶Hya1, is less deep in the bilayer than the residue in the other two peptides. This finding is supported by differential scanning calorimetry (DSC) data, which shows that both D⁰W⁶Hya1 and W⁶Hya1 disturb DPPC gel-fluid transition slightly more effectively than K⁰W⁶Hya1. DPPC DSC profiles are homogeneously disturbed by the three peptides, probably related to peptide-membrane diffusion. Surprisingly, the peptide that displays the lowest affinity for PC membranes and is located at the more superficial position in the bilayer, K⁰W⁶Hya1, is the most efficient in causing formation of pores on the membrane, as attested by carboxyfluorescein leakage assays. The three peptides were found to interact with CT DNA, with a deep penetration of the Trp residue into hydrophobic pockets of the double helix, as indicated by the significant blue shift on the Trp fluorescence, and the displacement of DNA-bound ethidium bromide by the peptides. The experiments of DNA electrophoresis confirm that Hylin peptides bind DNA in a concentration-dependent manner, inducing complete DNA retardation at the relative AMP/plasmid DNA weight ratio of ~17. These findings could help to better understand the AMPs toxic effects on eukaryotic cells, thus contributing to the design of healthier therapeutic agents.

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AMP Hylin a1 analogues bind to both a model of eukaryotic membrane and DNA.

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The most cationic peptide has lowest affinity to PC vesicle and shallower binding.

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Peptide lower bilayer affinity is related to greater vesicle disruption and leakage.

•AMP hylin a1 analogues deep penetrate into hydrophobic pockets of CT-DNA double helix.

Mechanistic insights into host adaptation, virulence and epidemiology of the phytopathogen Xanthomonas

Xanthomonas is a well-studied genus of bacterial plant pathogens whose members cause a variety of diseases in economically important crops worldwide. Genomic and functional studies of these phytopathogens have provided significant understanding of microbial-host interactions, bacterial virulence and host adaptation mechanisms including microbial ecology and epidemiology. In addition, several strains of Xanthomonas are important as producers of the extracellular polysaccharide, xanthan, used in the food and pharmaceutical industries. This polymer has also been implicated in several phases of the bacterial disease cycle. In this review, we summarise the current knowledge on the infection strategies and regulatory networks controlling virulence and adaptation mechanisms from Xanthomonas species and discuss the novel opportunities that this body of work has provided for disease control and plant health.

A Noncytotoxic Temporin L Analogue with In Vivo Antibacterial and Antiendotoxin Activities and a Nonmembrane-Lytic Mode of Action

Cytotoxic frog antimicrobial peptide Temporin L (TempL) is an attractive molecule for the design of lead antimicrobial agents due to its short size and versatile biological activities. However, noncytotoxic TempL variants with desirable biological activities have rarely been reported. TempL analogue Q3K,TempL is water-soluble and possesses a significant antiendotoxin property along with comparable cytotoxicity to TempL. A phenylalanine residue, located at the hydrophobic face of Q3K,TempL and the "d" position of its phenylalanine zipper sequence, was replaced with a cationic lysine residue. This analogue, Q3K,F8K,TempL, showed reduced hydrophobic moment and was noncytotoxic with lower antimicrobial activity. Interestingly, swapping between tryptophan at the fourth and serine at the sixth positions turned Q3K,F8K,TempL totally amphipathic as reflected by its helical wheel projection with clusters of hydrophobic and hydrophilic residues and the highest hydrophobic moment among these peptides. Surprisingly, this analogue, SW,Q3K,F8K,TempL, was as noncytotoxic as Q3K,F8K,TempL but showed augmented antimicrobial and antiendotoxin properties, comparable to that of TempL and Q3K,TempL. SW,Q3K,F8K,TempL exhibited appreciable survival of mice against P. aeruginosa infection and a lipopolysaccharide (LPS) challenge. Unlike TempL and Q3K,TempL, SW,Q3K,F8K,TempL adopted an unordered secondary structure in bacterial membrane mimetic lipid vesicles and did not permeabilize them or depolarize the bacterial membrane. Overall, the results demonstrate the design of a nontoxic TempL analogue that possesses clusters of hydrophobic and hydrophilic residues with impaired secondary structure and shows a nonmembrane-lytic mechanism and in vivo antiendotoxin and antimicrobial activities. This paradigm of design of antimicrobial peptide with clusters of hydrophobic and hydrophilic residues and high hydrophobic moment but low secondary structure could be attempted further.

Enhanced tolerance to Phytophthora root and stem rot by over-expression of the plant antimicrobial peptide CaAMP1 gene in soybean

BACKGROUND

Antimicrobial peptides play important roles in both plant and animal defense systems. Moreover, over-expression of CaAMP1 (Capsicum annuum antimicrobial protein 1), an antimicrobial protein gene isolated from C. annuum leaves infected with Xanthomonas campestris pv. vesicatoria, confers broad-spectrum resistance to hemibiotrophic bacterial and necrotrophic fungal pathogens in Arabidopsis. Phytophthora root and stem rot (PRR), caused by the fungus Phytophthora sojae, is one of the most devastating diseases affecting soybean (Glycine max) production worldwide.

RESULTS

In this study, CaAMP1 was transformed into soybean by Agrobacterium-mediated genetic transformation. Integration of the foreign gene in the genome of transgenic soybean plants and its expression at the translation level were verified by Southern and western blot analyses, respectively. CaAMP1 over-expression (CaAMP1-OX) lines inoculated with P. sojae race 1 exhibited enhanced and stable PRR tolerance through T2-T4 generations compared with the wild-type Williams 82 plants. Gene expression analyses in the transgenic plants revealed that the expression of salicylic acid-dependent, jasmonic acid-dependent, and plant disease resistance genes (R-genes) were significantly up-regulated after P. sojae inoculation.

CONCLUSIONS

These results indicate that CaAMP1 over-expression can significantly enhance PRR tolerance in soybean by eliciting resistance responses mediated by multiple defense signaling pathways. This provides an alternative approach for developing soybean varieties with improved tolerance against soil-borne pathogenic PRR.

Bee venom-derived antimicrobial peptide melectin has broad-spectrum potency, cell selectivity, and salt-resistant properties

Antimicrobial peptides have attracted attention as alternatives to conventional antibiotics. Previously, a novel antimicrobial peptide, melectin, consisting of 18 amino acids was isolated from the venom of a bee, Melecta albifrons. Here, we investigated the antibacterial activity of melectin against drug-resistant bacteria. Melectin showed broad-spectrum antimicrobial activity but low cytotoxicity and no hemolytic activity. Melectin maintained its antimicrobial activity at physiological salt concentrations. Melectin is an α-helical structure that binds to the bacterial membrane via electrostatic interactions and kills bacteria in a short time by bacterial membrane targeting. Collectively, our results suggest that melectin has antibacterial activity and anti-inflammatory activity.

Antimicrobial activity of RP-1 peptide conjugate with ferrocene group

Parasitic diseases are a neglected and serious problem, especially in underdeveloped countries. Among the major parasitic diseases, Leishmaniasis figures as an urgent challenge due to its high incidence and severity. At the same time, the indiscriminate use of antibiotics by the population is increasing together with resistance to medicines. To address this problem, new antibiotic-like molecules that directly kill or inhibit the growth of microorganisms are necessary, where antimicrobial peptides (AMPs) can be of great help. In this work, the ferrocene molecule, one active compound with low levels of in vivo toxicity, was coupled to the N-terminus of the RP1 peptide (derived from the human chemokine CXCL4), aiming to evaluate how this change modifies the structure, biological activity, and toxicity of the peptide. The peptide and the conjugate were synthesized using the solid phase peptide synthesis (SPPS). Circular dichroism assays in PBS showed that the RP1 peptide and its conjugate had a typical spectrum for disordered structures. The Fc-RP1 presented anti-amastigote activity against Leishmania amazonensis (IC₅₀ = 0.25 μmol L^–1). In comparison with amphotericin B, a second-line drug approved for leishmaniasis treatment, (IC₅₀ = 0.63 μmol L^-1), Fc-RP1 was more active and showed a 2.5-fold higher selectivity index. The RP1 peptide presented a MIC of 4.3 μmol L^-1 against S. agalactiae, whilst Fc-RP1 was four times more active (MIC = 0.96 μmol L^-1), indicating that ferrocene improved the antimicrobial activity against Gram-positive bacteria. The Fc-RP1 peptide also decreased the minimum inhibitory concentration (MIC) in the assays against E. faecalis (MIC = 7.9 μmol L^-1), E. coli (MIC = 3.9 μmol L^-1) and S. aureus (MIC = 3.9 μmol L^-1). The cytotoxicity of the compounds was tested against HaCaT cells, and no significant activity at the highest concentration tested (500 μg. mL^-1) was observed, showing the high potential of this new compound as a possible new drug. The coupling of ferrocene also increased the vesicle permeabilization of the peptide, showing a direct relation between high peptide concentration and high carboxyfluorescein release, which indicates the action mechanism by pore formation on the vesicles. Several studies have shown that ferrocene destabilizes cell membranes through lipid peroxidation, leading to cell lysis. It is noteworthy that the Fc-RP1 peptide synthesized here is a prototype of a bioconjugation strategy, but it still is a compound with great biological activity against neglected and fish diseases.

Dimerization of Antimicrobial Peptides: A Promising Strategy to Enhance Antimicrobial Peptide Activity

Antimicrobial resistance is a global health problem with strong social and economic impacts. The development of new antimicrobial agents is considered an urgent challenge. In this regard, Antimicrobial Peptides (AMPs) appear to be novel candidates to overcome this problem. The mechanism of action of AMPs involves intracellular targets and membrane disruption. Although the exact mechanism of action of AMPs remains controversial, most AMPs act through membrane disruption of the target cell. Several strategies have been used to improve AMP activity, such as peptide dimerization. In this review, we focus on AMP dimerization, showing many examples of dimerized peptides and their effects on biological activity. Although more studies are necessary to elucidate the relationship between peptide properties and the dimerization effect on antimicrobial activity, dimerization constitutes a promising strategy to improve the effectiveness of AMPs.

A scorpion venom peptide derivative BmKn‒22 with potent antibiofilm activity against Pseudomonas aeruginosa

Pseudomonas aeruginosa is a leading cause of nosocomial and serious life-threatening infections and infections caused by this bacterium continue to pose a major medical challenge worldwide. The ability of P. aeruginosa to produce multiple virulence factors and in particular to form biofilms makes this bacterium resistant to all known antibiotics. As a consequence, standard antibiotic therapy are increasingly become ineffective to clear such infections associated with biofilms. In search for novel effective agents to combat P. aeruginosa biofilm infections, a series of the BmKn‒2 scorpion venom peptide and its truncated derivatives were synthesized and their antibiofilm activities assessed. Among the peptides tested, BmKn‒22 peptide, which was a modified peptide of the parental BmKn‒2 scorpion venom peptide, clearly demonstrated the most potential inhibitory activity against P. aeruginosa biofilms without affecting the bacterial growth. This peptide was not only capable of inhibiting the formation of P. aeruginosa biofilms, but also disrupting the established biofilms of P. aeruginosa. Additionally, BmKn‒22 peptide was able to inhibit the production of key virulence factor pyocyanin of P. aeruginosa. Our results also showed that BmKn‒22 peptide significantly reduced lasI and rhlR expression, and suggested that BmKn‒22 peptide-mediated inhibition of P. aeruginosa biofilms and virulence factors was achieved through the components of quorum-sensing systems. Combination of BmKn‒22 peptide with azithromycin resulted in a remarkable reduction P. aeruginosa biofilms. Since this peptide exhibited low toxicity to mammalian cells, all our results therefore indicate that the BmKn‒22 peptide is a promising antibiofilm agent against P. aeruginosa and warrant further development of this peptide as a novel therapeutic for treatment of P. aeruginosa‒associated biofilm infections.

Self-Assembled Nanoparticles of Symmetrical Cationic Peptide Against Citrus Pathogenic Bacteria

The increasing drug resistance of phytopathogenic bacteria to conventional bactericides has driven the necessity for exploring new alternatives with a lower tendency to develop bacterial resistance. Here, we report a novel cationic symmetrical peptide P₅VP₅ (Ac- R+ LI R+ K+ V K+ R+ IL R+ -NH₂ that enables self-assembly to form nanoparticles with excellent thermal stability. An in vitro assay showed that P₅VP₅ nanoparticles exhibited excellent antibacterial activity against Xanthomonas axonopodis pv citri with a MIC value of 20 μM. Meanwhile, under an in planta condition, treatment with peptide nanoparticles demonstrated the highest ability to reduce the development of citrus canker lesions in leaves. Moreover, the nanoparticles could destroy the biofilm formation, damage the cell membranes, and affect the cell membrane permeability, ultimately leading to the death of bacteria. Taken together, these nanoparticles are a promising antibacterial agent that can be used to control citrus canker and other plant diseases caused by bacteria.